Chair base



FIG. 1 is a perspective view of a chair base showing my new design, the chair seat shown in broken lines for illustrative purposes only and forming no part of the claimed design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a right-side elevational view thereof;

FIG. 5 is a rear elevational view thereof; and,

FIG. 6 is a bottom view thereof. 

The ornamental design for a chair base, as shown and described. 